Digital-to-time converters (DTC) convert digital information to a time delay or a phase shift of the output signal. Therefore, digital-to-time converters can be used to generate a frequency- or phase-modulated clock in a polar transmitter.
Polar mode transmitters have recently become attractive thanks to a higher power efficiency compared to standard IQ (I=in-phase; Q=quadrature) transmitters. In the polar mode, the amplitude modulation is achieved by means of a DAC (DAC=digital-to-analog converter) or RFDAC (RFDAC=radio frequency digital-to-analog converter), while the phase information is delivered by modulating an input clock.
For low baseband frequencies, e.g., for GSM (GSM=global system for mobile communications) and UMTS (UMTS=universal mobile telecommunications system) standards, the modulation can be achieved by directly modulating a PLL (PLL=phase-locked loop) signal. For modern standards, such as LTE (LTE=long term evolution), the signal bandwidth is increased to 40 MHz and beyond (by carrier aggregation), making the PLL modulation ineffective. As a consequence, phase/frequency modulation is achieved by modulating a fixed frequency signal coming from the PLL by means of a DTC.
DTC integral/differential linearity is a key figure of merit for the performance of the converter. Large integral non-linearity generates harmonics in the output spectrum, while large differential non-linearity causes, in turn, a high noise floor. Identifying and reducing/eliminating sources of non-linearity in the DTC is therefore mandatory in order to achieve a clean modulated clock for the polar transmitter.